Despite favouring different geographic modes of speciation, biologists have differed considerably over the likely processes causing the initial genetic differentiation between neospecies. Recently, there has been a dogmatic shift away from non-adaptive processes towards adaptive selection as the major process. The shift is a result of both evidence against non-adaptive processes, such as founder effects, and evidence in favour of selection.
Evidence that counters the likelihood of a role for non-adaptive processes comes from both theory and empirical evidence. For example, Barton (1998) has argued from a population genetic viewpoint that population bottlenecks, such that make founder effects and drift likely, are also likely to decrease the chances of incipient species persistence due to lack of genetic variation. Theoretical examination of Wright's shifting balance has shown that it is possible to cross a non-adaptive valley only if the valley is shallow and population size is small. However, very shallow non-adaptive valleys are unlikely to persist in a variable environment, so there are just as likely to be episodes when selection can drive such a peak shift. Recent experiments on Drosophila have also failed to produce significant population differentiation from founder events (Rundle etal. 1998).
The pro-selection evidence comes from several sources (Schluter 1998). First, in some adaptive radiations, ecological divergence apparently occurs as rapidly as reproductive isolation. In many freshwater fish colonizing post-glacial lakes m y s
Allopatric speciation and rivers in the Northern Hemisphere, new sympatric sister species are strongly divergent ecologically, often with one benthic and one planktivo-rous species coexisting (Figure 9.5). There is strong evidence that these differences are adaptive, for they correlate with increased foraging efficiency which gives an advantage in competition with other fish. In addition, sometimes mate discrimination is the result of natural selection: in sticklebacks, body size is used as a trait in mate choice, but is also one of the major traits responsible for ecological divergence. In some cases, divergent selection can lead directly to post-zygotic isolation. Some monkeyflower (Mimulus gutta-tus) populations have adapted to soils contaminated with copper (Figure 7.5), but these alleles are lethal when combined with other alleles in hybrids with other populations that are not tolerant to copper (Chapter 7).
A second area of evidence comes from examining hybridization events. If differences between species are adaptive, hybrids should be less fit than their parents in the native habitat, but this difference should disappear in the lab. Genetic mechanisms reducing hybrid fitness, however, should be apparent even in the lab. In fact, many groups produce highly viable and fertile laboratory hybrids, including Drosophila, Darwin's finches, East African cichlids, Hawaiian silverswords, and fishes from post-glacial lakes mentioned above. A final source of evidence comes from measuring the rate of evolution across populations or species as compared to the null hypothesis of neutral evolution through mutation and drift. Studies by Orr (1998) showed that of eight loci affecting male genital structure in the sister species D. simulans and D. mauritiana, all eight indicated evolution by selection. Studies on the genetic variation in quantitative traits among populations also indicate divergent selection in many species (Schluter 2000).
Thus, there is now growing evidence that the origin of species is indeed by means of natural selection, as Darwin suggested in the title of his famous book (Darwin 1859). Furthermore, lineage splitting in sympatry is looking more likely in comparison to the dogma of previous decades, and sexual selection is increasingly invoked to play a role in species divergence.
What then, is the role of ecology in the formation of species? First, ecology sets the geographic stage for speciation of whatever kind. In sympatry, speciation requires disruptive selection, which must have an ecological dimension. For hybrid speciation geographic proximity is required, and hybrid fitness and spread is determined by the ecological situation. Even in allopatric cases, selection frequently creates the initial divergences between incipient species that can lead to reproductive isolation. If stochastic processes play a role, they too are dependent on specific ecological circumstances. Finally, natural selection is a major process driving further differentiation between species such that it becomes irreversible. Thus, the birth of species has a very fundamental ecological context. In the next chapter, we will see that the death of species, extinction, primarily a population ecology phenomenon, has an evolutionary context.
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